Conventionally, in an engine equipped with a float type carburetor, such as a general purpose small engine, if the engine is left for a long period of time in an unused state with fuel remaining in a float chamber of the carburetor, the residual fuel gradually oxidizes and forms a gum within the float chamber, the fuel clogs a main jet or a breather hole, thus causing engine starting faults or poor running, and there is also the problem that when the engine is tilted the residual fuel flows into an intake passage through a nozzle.
In order to solve such problems, conventionally a drain plug is provided in a lower part of the carburetor, and after the engine is used or before it is stored the drain plug is manually operated so as to drain the residual fuel, but such an operation is not only troublesome and difficult but also undesirable in terms of the environment because of contamination of the surroundings of the engine, which is a problem.
Automatic residual fuel vent means have already been disclosed in, for example, Patent Publications 1 and 2 below, in which fuel within a float chamber of a carburetor is automatically vented by utilizing intake negative pressure of an engine before the engine is stopped, and is returned to a fuel tank.    Patent Publication 1: Japanese Utility Model Registration Publication No. 60-27808    Patent Publication 2: Japanese Patent Publication No. 1-59427